1. Field of the Invention
This invention relates to a 1-phase energized disk-type brushless motor having a single position detecting element.
2. Description of the Prior Art
As various systems have been developed, brushless motors, especially disk-type brushless motors, suitable for use with such systems, have been demanded. Disk-type brushless motors can be used as disk-type brushless fan motors which are widely used in office machines and the like, and in some applications, they are required to be very inexpensive, small and very flat.
Those motors which meet the requirements best are 1-phase energized brushless motors wherein only one position detecting element is required. It is to be noted that while a motor of the type is sometimes called a 2-phase motor from the number of armature coils (for example, 2, 4, etc.), it should be called exactly a 1-phase motor from the energizing method. However, such a 1-phase (single phase) motor has a dead point at an energization switching point at which the motor provides zero torque. Accordingly, the 1-phase motor has a drawback that it cannot start itself if the rotor position upon starting of the motor is just at a dead point and hence the position detecting element detects a boundary between the N (north) pole and the S (south) pole of the magnet rotor, that is, the dead point.
Accordingly, a 1-phase motor normally includes a cogging generating magnetic member (an iron piece is used therefor) foro generating a torque (cogging torque) in addition to a torque generated by an armature coil and a magnet rotor in order to eliminate such dead points to allow self-starting of the motor.
In a coreless motor, for example, following methods for generating a cogging torque are known. Referring first to FIG. 1, a 6-pole magnet rotor (field magnet) 2 having an alternate arrangement of the 6 north and south poles is mounted on a rotor yoke 1 in an opposing relationship to a stator yoke 5 with an air gap 4 left therebetween and with a pair of coreless armature coils 3 disposed in the air gap 4. In the motor of FIG. 1, the stator yoke 5 has at a fact thereof opposing the magnet rotor (hereinafter referred to as field magnet) 2 two inclined surfaces which thus define the complementarily inclined air gap 4. This method, however, has a drawback that the efficiency is relatively low because the air gap is relatively great.
Referring now to FIG. 2, another method is illustrated. In the motor of FIG. 2, a stator yoke 5 has no such inclined faces are provided on the stator yoke 5 of FIG. 1. Instead, an iron bar 6 is mounted on the stator yoke 5 and extends through each of a pair of coreless armature coils 3 disposed in a uniform air gap 4 defined by the stator yoke 5 and a field magnet 2 on a rotor yoke 1. According to this arrangement, a magnetic flux will appear as seen in FIG. 3 and hence the field magnet 2 will stop at a position in which the iron bars 6 are each opposed to the center of one of the N and S poles of the field magnet 2. Accordingly, if the armature coils 3 are located so as to produce a rotational torque in such a stopping position of the field magnet 2, a coreless motor which can start itself will be obtained.
However, the method as shown in FIG. 2 has a drawback that if the thickness of the iron bars 6 is increased in order to increase the cogging torque, a phenomenon that the torque around dead points decreases will appear because a magnetic flux 7 will act as shown in FIG. 4 around the dead points.